Semiconductor device having isolation structures with different thicknesses

ABSTRACT

A semiconductor structure includes a semiconductor substrate, a first fin, a second fin, a first isolation structure, and a second isolation structure. The semiconductor substrate has a memory device region and a logic core region. The first fin is in the memory device region of the semiconductor substrate. The second fin is in the logic core region of the semiconductor substrate. The first isolation structure is around the first fin. The second isolation structure is around the second fin, and a thickness of the first isolation structure is different from a thickness of the second isolation structure.

PRIORITY CLAIM AND CROSS-REFERENCE

This is a continuation application of the U.S. application Ser. No.15/660,107, filed Jul. 26, 2017, now U.S. Pat. No. 10,157,770, issuedDec. 18, 2018, which claims priority to U.S. Provisional ApplicationSer. No. 62/427,056, filed Nov. 28, 2016, which is herein incorporatedby reference.

BACKGROUND

Shallow trench isolation (STI) helps to prevent electrical currentleakage between adjacent semiconductor devices. In STI, one or moretrenches, i.e., the trenches, are etched into a surface of a substrateand then filled with a dielectric material. The trenches are used toisolate semiconductor devices. The dielectric material helps to reduceelectrical current leakage between the semiconductor devices.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIGS. 1-14 are cross-sectional views of a method for manufacturing anintegrated circuit at various stages in accordance with some embodimentsof the present disclosure.

FIG. 15 is a top view of an integrated circuit in accordance with someembodiments of the present disclosure.

FIGS. 16-28 are cross-sectional views of a method for manufacturing anintegrated circuit at various stages in accordance with some embodimentsof the present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

FIGS. 1-14 are cross-sectional views of a method for manufacturing anintegrated circuit at various stages in accordance with some embodimentsof the present disclosure. As shown in FIG. 1, semiconductor substrate110 includes portions in device regions 112, 114, 116 and 118. In someembodiments, the semiconductor substrate 110 includes silicon. Othermaterials, such as carbon, germanium, gallium, arsenic, nitrogen,indium, phosphorus, or the like, may also be included in semiconductorsubstrate 110. In some embodiments, device regions 112, 114, 116 and 118are different regions exemplarily including a logic core region, a highvoltage (HV) device region, a memory device region (such as an embeddednon-volatile memory (NVM) region or an embedded static random accessmemory (SRAM) region), a complementary metal-oxide-semiconductor (CMOS)image sensor region, an analog region, an input/output region, a dummyregion (for forming dummy patterns), or the like. The above-referenceddevice regions are schematically illustrated in FIG. 15. In someexemplary embodiments, the device region 112 is a logic core region, thedevice region 114 is a HV device region, the device region 116 is amemory device region, and the device region 118 is a CMOS image sensorregion.

Pad layer 120 and mask layer 130 are formed on semiconductor substrate110. The pad layer 120 is blanket formed on the semiconductor substrate110, and it may be a thin film comprising silicon oxide formed, forexample, using a thermal oxidation process, a deposition process, suchas chemical vapor deposition (CVD), physical vapor deposition (PVD), orother suitable processes. The mask layer 130 is blanket formed on thepad layer 120. The pad layer 120 acts as an adhesion layer betweensemiconductor substrate 110 and mask layer 130. The pad layer 120 mayalso act as an etch stop layer for etching mask layer 130. In someembodiments, the mask layer 130 is formed of silicon nitride, forexample, using low-pressure chemical vapor deposition (LPCVD). In otherembodiments, the mask layer 130 is formed by thermal nitridation ofsilicon, plasma enhanced chemical vapor deposition (PECVD), or plasmaanodic nitridation. The mask layer 130 is used as a hard mask duringsubsequent photolithography processes. For example, the mask layer 130may be used to protect the substrate 110 from fabrication operationsinvolved in the etching of trenches in the substrate 110 and subsequentchemical mechanical polishing (CMP) planarization operations.

Mask 140 is formed over the mask layer 130. The mask 140 may be aphotoresist, and it can be patterned with openings, such as openings 141and 142, in the mask 140 corresponding to locations of trenches to becreated. For example, a layer of photoresist material is deposited overthe semiconductor substrate 110. The layer of photoresist material isirradiated (exposed) in accordance with a predetermined pattern anddeveloped to remove portions of the photoresist material, so as to formthe openings 141 and 142. The remaining photoresist material protectsthe underlying material from subsequent processing step performed inFIG. 2, such as etching.

Reference is made to FIG. 2. Portions of the semiconductor substrate 110underlying the openings 141 and 142 are removed or recessed to formfirst trenches 151 and 152, and semiconductor fin F1 is also formedbetween the first trenches 151 and 152. That is, the first trenches 151and 152 are etched into the semiconductor substrate 110, and a portionof semiconductor substrate 110 between first trenches 151 and 152 thusbecomes the semiconductor fin F1 protruding from a portion of thesemiconductor substrate 110 underlying the first trenches 151 and 152.The semiconductor fin F1 may be used to form one or more semiconductordevices, such as CMOS image sensors, on the device region 118. Theetching of the first trenches 151 and 152 may be performed using any ofa variety of substrate etching techniques, such as plasma etching at avariety of pressures, temperatures, and so forth. The etching techniquemay also etch through the mask layer 130 and the pad layer 120. Sincethe first trenches 151 and 152 are etched using the same etching processand at the same time, the first trenches 151 and 152 can havesubstantially the same depth, i.e., a first depth D1.

Reference is made to FIG. 3. The mask 140 in FIG. 2 is removed, forexample, using an ashing process. Next, mask 160 is applied to thesubstrate 110 to protect the trenches 151 and 152 already created on thesubstrate 110. The mask 160 may be a photoresist. This photoresist isapplied to an entirety of the substrate 110 and then patterned so thatportions of the mask 160 over parts of the substrate 110 with firsttrenches 151 and 152 remain. A pattern is also applied to the mask 160,wherein the pattern is for use in creating trenches of second depthsdifferent from the first depths D1 of the first trenches 151 and 152.The pattern includes openings 161, 162, 163, 164, 165 and 166 in themask 160. For example, the photoresist may be patterned by exposure anddevelopment as discussed previously to form the openings 161-166,wherein none of the openings 161-166 overlie the first trenches 151 and152. The remaining photoresist material protects the underlying materialfrom subsequent processing step performed in FIG. 4, such as etching.

Reference is made to FIG. 4. With the pattern of the mask 160 includingthe openings 161-166 is created, second trenches 171, 172, 173, 174, 175and 176 corresponding to the openings 161-166 are etched into thesubstrate 110. In other words, portions of the semiconductor substrate110 are removed or recessed to form the second trenches 171-176, andsemiconductor fins F2, F3, F4, F5 are formed as well. For example, thesecond trenches 171 and 172 are etched into the semiconductor substrate110, and a portion of the semiconductor substrate 110 between the secondtrenches 171 and 172 thus becomes the semiconductor fin F2 protrudingfrom a portion of the semiconductor substrate 110 underlying the secondtrenches 171 and 172. The second trenches 173-176 and the semiconductorfins F3-F5 can be formed in a similar manner. The semiconductor fin F2may be used to form one or more semiconductor devices, such as logicdevices, on the device region 112. The semiconductor fin F3 may be usedto form one or more semiconductor devices, such as HV devices, on thedevice region 114. The semiconductor fins F4 and F5 may be used to formone or more semiconductor devices, such as memory devices, on the deviceregion 116. The etching of the second trenches 171-176 may be performedusing any of a variety of substrate etching techniques, such as plasmaetching at a variety of pressures, temperatures, and so forth. Theetching technique may also etch through the mask layer 130 and the padlayer 120. Since the second trenches 171-176 are etched using the sameetching process and at the same time, the second trenches 171-176 canhave substantially the same depth, i.e. a second depth D2.

As shown in FIG. 4, the second depths D2 of the second trenches 171-176are different from the first depths D1 of the first trenches 151 and152. For example, as illustrated, the first depths D1 of the firsttrenches 151 and 152 are greater than the second depths D2 of the secondtrenches 171-176. In other words, the first trenches 151 and 152 aredeeper than the second trenches 171-176. A difference between the firstdepth D1 and the second depth D2 may range from about 100 nm and about3000 nm. The second trenches 171-176 have bottoms located at a heightdifferent from a height of the bottoms of the first trenches 151 and152. For example, the bottoms of the first trenches 151 and 152 arelocated at a height lower than a height of the bottoms of the secondtrenches 171-176. The depth difference between the first trenches151-152 and the second trenches 171-176 can be formed using differentetching parameters to etch the first trenches 151-152 and the secondtrenches 171-176. That is, the etching parameters for etching the firsttrenches 151 and 152 are different from that for etching the secondtrenches 171-176, so as to create a predetermined depth difference. Thesecond trenches 175 and 176 expose opposite sidewalls of the mask 160protecting the first trenches 151 and 152, and therefore, the secondtrenches 175 and 176 may be adjacent to the first trenches 151 and 152,respectively. Stated differently, the second trench 175 and the firsttrench 151 deeper than the second trench 175 are communicated after themask 160 is removed. Similarly, the second trench 176 and the firsttrench 152 deeper than the second trench 176 are communicated after themask 160 is removed as well.

Reference is made to FIG. 5. The mask 160 in FIG. 4 is removed, forexample, using an ashing process. Next, a mask 180 is applied to thesubstrate 110 to protect the first trenches 151-152 and second 171-176already created on the substrate 110. The mask 180 may be a photoresist.This photoresist is applied to an entirety of the substrate 110 and thenpatterned so that portions of the mask 180 over parts of the substrate110 with first trenches 151-152 and second 171-176 remain. A pattern isalso applied to the mask 180, wherein the pattern is for use in creatingtrenches of third depths different from the first depths D1 of the firsttrenches 151 and 152 and the second depths D2 of the second trenches171-176. The pattern includes openings 181, 182 and 183 in the mask 180.For example, the photoresist can be patterned by exposure anddevelopment as discussed previously to form the openings 181-183,wherein none of the openings 181-183 overlie the first trenches 151, 152and second trenches 171-176. The remaining photoresist material protectsthe underlying material from subsequent processing step performed inFIG. 6, such as etching.

Reference is made to FIG. 6. With the pattern of the mask 180 includingopenings 181-183 is created, third trenches 191, 192 and 193corresponding to the openings 181-183 may be etched into the substrate110. In other words, portions of the semiconductor substrate 110 areremoved or recessed to form the third trenches 191-193, andsemiconductor fins F6 and F7 are formed as well. For example, the thirdtrenches 191, 192 and 193 are etched into the semiconductor substrate110, a portion of the semiconductor substrate 110 between the thirdtrenches 191 and 192 thus becomes the semiconductor fin F6 protrudingfrom a portion of the semiconductor substrate 110 underlying the thirdtrenches 191 and 192, and another portion of the semi conductorsubstrate 110 between the third trenches 192 and 193 thus becomes thesemiconductor fin F7 protruding from a portion of the semiconductorsubstrate 110 underlying the third trenches 192 and 193. Thesemiconductor fins F6 and F7 may be used to form semiconductor devices,such as logic devices, on the device region 112. The etching of thethird trenches 191-193 may be performed using any of a variety ofsubstrate etching techniques, such as plasma etching at a variety ofpressures, temperatures, and so forth. The etching technique may alsoetch through the mask layer 130 and the pad layer 120. Since the thirdtrenches 191-193 are etched using the same etching process and at thesame time, the third trenches 191-193 can have substantially the samedepth, i.e. a third depth D3.

As shown in FIG. 6, the third depths D3 of the third trenches 191-193are different from the first depths D1 of the first trenches 151 and 152and the second depths D2 of the second trenches 171-176. For example, asillustrated, the second depths D2 of the second trenches 171-176 aregreater than the third depths D3 of the third trenches 191-193. Thefirst, second and third depths D1, D2 and D3 may satisfy: D1>D2>D3. Inother words, the second trenches 171-176 are deeper than the thirdtrenches 191-193. A difference between the second depth D2 and the thirddepth D3 may range from about 5 nm to about 120 nm. The depth differencebetween the second trenches 171-176 and the third trenches 191-193 canbe formed using different etching parameters to etch the second trenches171-176 and the third trenches 191-193. That is, the etching parametersfor etching the second trenches 171-176 are different from that foretching the third trenches 191-193, so as to create a predetermineddepth difference. The third trench 193 exposes a sidewall of the mask180 protecting the second trench 172, and therefore, the third trench193 may be adjacent to the second trench 172. Stated differently, thethird trench 193 and the second trench 172 are communicated after themask 180 is removed. After the third trenches 191-193 are created, themask 180 is removed, for example, using an ashing process. If additionaltrenches are to be created in the substrate 110, additional patterns maybe created and additional etching operations may be repeated to createthe additional trenches.

Reference is made to FIG. 7. Dielectric feature 200 is then formed onthe substrate 110 to cover the semiconductor fins F1 and F3-F7 and tofill the trenches 151, 152, 171-176 and 191-193. The dielectric feature200 includes a material such as silicon oxide, silicon nitride, siliconoxynitride, low-k materials, other suitable materials, or anycombinations thereof. In some embodiments that the dielectric feature200 includes silicon oxide, the silicon oxide can be formed by CVD,atomic layer deposition (ALD), high density plasma CVD (HDPCVD), othersuitable methods, or combinations thereof. The silicon oxide may bealternatively formed by a high aspect ratio process (HARP). In someembodiments, an optional thermal oxide trench liner is grown to improvethe trench interface. The CVD process for depositing the dielectricfeature 200, for example, can use chemicals including Hexachlorodisilane(HCD or Si₂Cl₆), Dichlorosilane (DCS or SiH₂Cl₂),Bis(TertiaryButylAmino)Silane (BTBAS or C₈H₂₂N₂Si) and Disilane (DS orSi₂H₆). In some embodiments, the dielectric feature 200 can have amulti-layer structure, for example, a thermal oxide liner layer withsilicon nitride formed over the liner. Thereafter, a thermal annealingwill be performed to the dielectric feature 200.

Next, as shown in FIG. 8, planarization process, such as CMP process isperformed to remove excess dielectric feature 200 outside the trenches151, 152, 171-176 and 191-193. The planarization process may also removethe pad layer 120 and the mask layer 130 such that the semiconductorfins F1 and F3-F7 are exposed. After the planarization, portions of thedielectric feature 200 filling the first trenches 151 and 152 can bereferred to first shallow trench isolations (STIs) 201 and 202, portionsof the dielectric feature 200 filling the second trenches 171-176 can bereferred to as second STIs 211-216, and portions of the dielectricfeature 200 filling the third trenches 191-193 can be referred to asthird STIs 221-223. These STIs can be referred to as isolationstructures in some embodiments.

The planarization process may reduce the first, second and third depthsD1, D2 and D3 to first, second and third depth D1′, D2′ and D3′,respectively. That is, after the planarization process, the firsttrenches 151 and 152 have reduced first depth D1′, the second trenches171-176 have reduced second depth D2′, and the third trenches 191-193have reduced third depth D3′. The first STIs 201 and 202 filling thefirst trenches 151 and 152 may have substantially the same thickness,which is substantially equal to the first depth D1′. The second STIs211-216 filling the second trenches 171-176 may have substantially thesame thickness, which is substantially equal to the second depth D2′.The third STIs 221-223 filling the third trenches 191-193 may havesubstantially the same thickness, which is substantially equal to thethird depth D3′. Since the planarization process form a substantialplanar surface for the structure shown in FIG. 8, the reduced first,second and third depth D1′, D2′ and D3′ may satisfy: D1′>D2′>D3′, whichis similar to D1>D2>D3 as discussed above. Therefore, thicknesses of thefirst STIs 201 and 202 are greater than thicknesses of the second STIs211-216, and thicknesses of the second STIs 211-216 are greater thanthicknesses of the third STIs 221-223 at this stage. Such thicknessdifferences may be advantageous to provide various isolations suitablefor different device regions 112-118 that have different functions. Forexample, semiconductor devices subsequently formed on the device regions112-118 for providing different functions may have different devicecharacteristics, such as device dimensions, driving currents, thresholdvoltages, device densities, and so forth. STIs having differentthicknesses are thus advantageous to provide suitable isolations for thedevice regions 112-118. Stated differently, different STI thicknessesmay allow for optimization of the junction isolation for differentsemiconductor devices in an integrated circuit.

In some embodiments, the first STI 201 and second STI 215 respectivelyfilling the first trench 151 and the second trench 175 that arecommunicated with each other, and hence the first STI 201 abuts thesecond STI 215, and the first STI 201 is thicker than the second STI215. Stated differently, the first STI 201 and second STI 215 thinnerthan the first STI 201 are monolithically connected, immediatelyadjacent, or in contact with each other, and materials other than theSTIs 201 and 215 are absent between the STIs 201 and 215. For example, asemiconductor feature, a conductive feature or a combination thereof isabsent between the STIs 201 and 215. For example, in some embodiments,the first STI 201 has a sidewall S1, and the second STI 215 thinner thanthe first STI 201 abuts an upper region of the sidewall S1. Stateddifferently, the first STI 201 has a portion protruded from a bottomsurface of the second STI 215. This arrangement may be advantageous toreduce the distance between semiconductor devices on the adjacent deviceregions 116 and 118 using different STI thicknesses. For example, thedevice regions 116 and 118 respectively have semiconductor fins F5 andF1 thereon, and the semiconductor fins F5 and F1 are adjacent, whichmeans that an additional semiconductor fin is absent between thesemiconductor fins F5 and F1. The STI isolating the semiconductor finsF5 and F1 may include the first STI 201 abutting the semiconductor finF1 and the second STI 215 abutting the semiconductor fin F5, and thefirst and second STIs 201 and 215 are abutted as well.

In some embodiments, the first STIs 201 and 202 respectively have bottomsurfaces at a height different from a height of the bottom surfaces ofthe second STIs 211-216, and the bottom surfaces of the second STIs211-216 are located at a height different from heights of the bottomsurfaces of the third STIs 221-223. For example, the bottom surface ofthe first STI 201 is located at the height lower than the height of itsneighboring second STI 215, or stated differently, the bottom surface ofthe first STI 201 is in a position lower than the bottom surface of thesecond STI 215. This bottom height difference may be beneficial to makethe first STI 201 thicker than the second STI 215. Stated differently,the STI isolating the adjacent semiconductor fins F1 and F5 may includean STI W1 and a dielectric protrusion P1. The dielectric protrusion P1protrudes from a bottom of the STI W1, and the dielectric protrusion P1has a width less than the width of the STI W1. The dielectric protrusionP1 protrudes from the STI W1 toward the substrate 110. The dielectricprotrusion P1 is closer to the semiconductor fin F1 than to thesemiconductor fin F5. For example, the dielectric protrusion P1 abutsthe semiconductor fin F1 and is spaced apart from the semiconductor finF5. Stated differently, the isolation structure isolating the adjacentsemiconductor fins F1 and F5 includes a first portion (i.e. STI 201) anda second portion (i.e. STI 215). The first portion is closer to thesemiconductor fin F1 than the second portion, and the first portion isthicker than the second portion. For example, the isolation structureisolating two neighboring fins has a stepped bottom surface. Using thisconfiguration, a portion of the STI adjacent to the semiconductor fin F1has a thickness different from that adjacent to the semiconductor finF5. This thickness difference may be advantageous to provide suitableisolations for the adjacent semiconductor fins F1 and F5.

Reference is made to FIG. 9. Sacrificial layer 230 is formed on at leastthe semiconductor fins F1 and F3-F7. The sacrificial layer 230 may beused for implantation screening and reduction of the channeling effectduring the subsequent implantation. The sacrificial layer 230 may be anoxide layer formed, for example, using CVD or PVD. Next, ionimplantation process is performed to impart impurities to thesemiconductor fins F1 and F3-F7 and to form wells in the semiconductorsubstrate 110.

Reference is made to FIG. 10. The sacrificial layer 230 is removed, andthen mask 240 is applied to mask or cover a portion of the substrate110, leaving another portion of the substrate 110 exposed. The mask 240may be a photoresist. This photoresist is applied to an entirety of thesubstrate 110 and then patterned so that portions of the mask 240 overparts of the substrate 110 with STIs 201,202 and 214-216 remain. TheSTIs 211, 212 and 221-223 are exposed by the mask 240. In someembodiments, a portion of the second STI 213 is covered, and a portionof the third STI 213 is exposed.

The exposed STIs are then recessed through an etching process untilupper portions of the semiconductor fins F3, F6 and F7 are exposed,resulting in recessed or lowered fourth STIs 251, 252, 253, and fifth261, 262 and 263 on the semiconductor substrate 110, and the resultingstructure is shown in FIG. 11. As illustrated, the second STIs 211 and212 are recessed to form the recessed fourth STIs 251 and 252, and thethird STIs 221-223 are recessed to form the recessed fifth STIs 261-263.In some embodiments, an unmasked portion of the second STI 213 betweensemiconductor fins F3 and F4 is recessed to form the recessed fourth STI253, and a masked portion of the STI 213 is not recessed by this etchingprocess. In some embodiments, the etching process may be a wet etchingprocess, for example, by dipping the substrate 110 in hydrofluoric acid(HF). In alternative embodiments, the etching process may be a dryetching process, for example, the dry etching process may be performedusing CHF₃ or BF₃ as etching gases. Since the STIs 251-253 and 261-263are recessed using the same etching process and at the same time, theSTIs 251-253 and 261-263 can have top surfaces at substantially the sameheight. After the etching process, the mask 240 is removed. In someembodiments, a ratio of the thickness of fifth STI 263 to the thicknessof the fourth STI 252 abutting the fifth STI 263 ranges from about 0.3to about 0.8.

Reference is made to FIG. 12, mask 270 is applied to mask or cover aportion of the substrate 110, leaving another portion of the substrate110 exposed. The mask 270 may be a photoresist. This photoresist isapplied to an entirety of the substrate 110 and then patterned so thatportions of the mask 270 over parts of the substrate 110 with recessedSTIs 251-253 and 261-263 remain. The non-recessed STIs 201, 202 and213-216 are exposed by the mask 270.

The exposed STIs are then recessed through an etching process untilupper portions of the semiconductor fins F1, F4 and F5 are exposed,resulting in recessed or lowered sixth STIs 281, 282, 283, and 284, andseventh STIs 291 and 292 on the semiconductor substrate 110, and theresulting structure is shown in FIG. 13. As illustrated, the second STIs214-216 are recessed to form the recessed sixth STIs 282-284, and thefirst STIs 201 and 202 are recessed to form the recessed seventh STIs291 and 292. A remaining portion of the second STI 213 is recessed toform the recessed sixth STI 281. In some embodiments, the etchingprocess may be a wet etching process, for example, by dipping thesubstrate 110 in hydrofluoric acid (HF). In alternative embodiments, theetching process may be a dry etching process, for example, the dryetching process may be performed using CHF₃ or BF₃ as etching gases.Since the STIs 281-284 and 291-292 are recessed using the same etchingprocess and at the same time, the STIs 281-284 and 291-292 can have topsurfaces at substantially the same height. In some embodiments, the STIs281-284 and 291-292 have top surfaces at the height different from aheight of the top surfaces of the STIs 251-253 and 261-263. For example,the top surfaces of the STIs 281-284 and 291-292 may be located inpositions higher than the top surfaces of the STIs 251-253 and 261-263.This top height difference can be controlled using etching parametersfor STIs 281-284 and 291-292 different from that for STIs 251-253 and261-263. In some embodiments, a ratio of the thickness of sixth STI 283to the thickness of the seventh STI 291 abutting the sixth STI 283ranges from about 1/12 to about 1/24.

In some embodiments, different portions of the second STI 213 betweenthe semiconductor fins F3 and F4 is recessed through different etchingprocesses, and therefore, the resulting recessed fourth STI 253 andsixth STI 281 between the adjacent semiconductor fins F3 and F4 can havedifferent thicknesses. For example, the sixth STI 281 has a thicknessgreater than the thickness of the fourth STI 253, and the fourth STI 253abuts the sixth STI 281. The thickness difference can be controlledusing different etching parameters to etch the different portions of theSTI 213. In some embodiments, the top surface of the fourth STI 253 islocated in a position lower than the top surface of the sixth STI 281.The top height difference is beneficial to form the STIs 253 and 281with different thicknesses. In some embodiments, a portion of thesemiconductor fin F3 protrudes from the fourth STI 253 and can bereferred to as a protrusion FP1, and a portion of the semiconductor finF4 protrudes from the sixth STI 281 and can be referred to as aprotrusion FP2. The protrusions FP1 and FP2 have different thicknessesdue to the thickness difference between the STIs 253 and 281. In someembodiments, a ratio of the thickness of fourth STI 253 to the thicknessof the sixth STI 281 ranges from about 1/1.2 to about 1/1.6.

The STIs 253 and 281 are abutted, monolithically connected, immediatelyadjacent, or in contact with to each other, and materials other than theSTIs 253 and 281 are absent between the STIs 253 and 281. For example, asemiconductor feature, a conductive feature or a combination thereof isabsent between the STIs 253 and 281. This arrangement may beadvantageous to reduce the distance between semiconductor devices on theadjacent device regions 114 and 116 using different STI thicknesses. Forexample, the device regions 114 and 116 respectively have semiconductorfins F3 and F4 thereon, and the semiconductor fins F3 and F4 areadjacent, which means that an additional semiconductor fin is absentbetween the semiconductor fins F3 and F4. The STI isolating thesemiconductor fins F3 and F4 may include the sixth STI 281 abutting thesemiconductor fin F4 and the fourth STI 253 abutting the semiconductorfin F3. The sixth STI 281 abuts the fourth STI 253 as well. For example,the sixth STI 281 has a sidewall S2, and the fourth STI 253 abuts alower region of the sidewall S2 of the sixth STI 281. Stateddifferently, the sixth STI 281 has a portion protruded from a topsurface of the fourth STI 253. The abutted STIs 253 and 281 havingdifferent thicknesses may provide suitable isolations for the adjacentsemiconductor fins F3 and F4, which are used to form semiconductordevices having different functions.

In some embodiments, the STI isolating the adjacent semiconductor finsF3 and F4 may include an STI W2 and a dielectric protrusion P2. Thedielectric protrusion P2 protrudes from a top of the STI W2, and thedielectric protrusion P2 has a width less than the width of the STI W2.The dielectric protrusion P2 protrudes from the STI W2 in a directionfarther away from the substrate 110. The dielectric protrusion P2 iscloser to the semiconductor fin F4 than to the semiconductor fin F3. Forexample, the dielectric protrusion P2 abuts the semiconductor fin F4 andis spaced apart from the semiconductor fin F3. Stated differently, theisolation structure isolating the adjacent semiconductor fins F3 and F4includes a first portion (i.e. STI 281) and a second portion (i.e. STI253). The first portion is closer to the semiconductor fin F4 than thesecond portion, and the first portion is thicker than the secondportion. For example, the isolation structure isolating two neighboringfins has a stepped top surface. Using this configuration, a portion ofthe STI adjacent to the semiconductor fin F4 has a thickness differentfrom that adjacent to the semiconductor fin F3. This thicknessdifference may be advantageous to provide suitable isolations for theadjacent semiconductor fins F3 and F4.

In some embodiments, top surfaces of the STIs 253 and 291 are located atdifferent heights, and bottom surfaces of the STIs 253 and 291 arelocated at different heights as well. Such differences may providefurther different isolations suitable for the semiconductor fins F1 andF3. In some embodiments, a height difference between the top surfaces ofthe STIs 253 and 291 is different from that between the bottom surfacesof the STIs 253 and 291. Such a difference may provide further differentisolation structures having different thicknesses suitable for thesemiconductor fins F1 and F3.

FIG. 14 illustrates the formation of gate structures 310, 320, 330 and340 in device regions 112, 114, 116 and 118, respectively. The gatestructures 320 and 330 have bottom surfaces at different heights due tothat the STIs 253 and 281 respectively underlie the gate structures 320and 330 having top surfaces at different heights. For example, thebottom surface of the gate structure 320 is lower than the bottomsurface of the gate structure 330. Gate dielectric 312 of the gatestructure 310 is formed to wrap the semiconductor fins F6 and F7 on thelogic device region 112. Gate electrode 314 of the gate structure 310 isthen formed on gate dielectric 312. Gate dielectric 322 of the gatestructure 320 is formed to wrap the semiconductor fin F3 on the HVdevice region 114. Gate electrode 324 of the gate structure 320 is thenformed on gate dielectric 322. Gate dielectric 332 of the gate structure330 is formed to wrap the semiconductor fins F4 and F5 on the memorydevice region 116. Gate electrode 334 of the gate structure 330 is thenformed on gate dielectric 332. Gate dielectric 342 of the gate structure340 is formed to wrap the semiconductor fin F1 on the CMOS image sensorregion 118. Gate electrode 344 of the gate structure 340 is then formedon gate dielectric 342.

In some embodiments, gate dielectrics 312-342 may include dielectricmaterials, such as high-k dielectric materials, suitable for thecorresponding device regions 112-118 formed by a suitable technique,such as CVD or PVD. Gate electrodes 314-344 may include conductors, suchas metals, suitable for the corresponding device regions 112-118 formedby a suitable technique, such as CVD or PVD. Source/drain regions forthe semiconductor fins F1 and F3-F7 can be formed before or after theformation of the gate structures 310-340, for example, using a suitabletechnique, such as implant processes or combinations of epitaxyprocesses and implant processes. As a result, semiconductor devices canbe formed on the device regions 112-118.

FIGS. 16-28 are cross-sectional views of a method for manufacturing anintegrated circuit at various stages in accordance with some embodimentsof the present disclosure. As shown in FIG. 16, semiconductor substrate410 includes portions in device regions 412, 414, 416 and 418. In someembodiments, device regions 412, 414, 416 and 418 are different regionsexemplarily including a logic core region, a high voltage (HV) deviceregion, a memory device region (such as an embedded non-volatile memory(NVM) region or an embedded static random access memory (SRAM) region),a CMOS image sensor region, an analog region, an input/output region, adummy region (for forming dummy patterns), or the like. Theabove-referenced device regions are schematically illustrated in FIG.15. In some exemplary embodiments, the device region 412 is a logic coreregion, the device region 414 is a HV device region, the device region416 is a memory device region, and the device region 418 is a CMOS imagesensor region.

Pad layer 420 and mask layer 430 are formed on semiconductor substrate410. The pad layer 420 may be a thin film comprising silicon oxideformed, for example, using a thermal oxidation process, a depositionprocess, such as chemical vapor deposition (CVD), physical vapordeposition (PVD), or other suitable processes. The pad layer 420 acts asan adhesion layer between semiconductor substrate 410 and mask layer430, and it can be formed of silicon nitride, for example, usinglow-pressure chemical vapor deposition (LPCVD), thermal nitridation ofsilicon, plasma enhanced chemical vapor deposition (PECVD), or plasmaanodic nitridation.

Mask 440 is formed over the mask layer 430. As shown in FIG. 16, themask 440 may be a photoresist, and it can be patterned by exposure anddevelopment, with openings in the mask 440 corresponding to locations oftrenches to be created. The remaining photoresist material protects theunderlying material from subsequent processing step performed in FIG.17, such as etching.

Reference is made to FIG. 17. Unmasked portions of the semiconductorsubstrate 410 are removed or recessed to form first trenches 451-457,and semiconductors F11-F16 are formed as well. For example, the firsttrenches 451 and 452 are etched into the semiconductor substrate 410,and a portion of semiconductor substrate 410 between first trenches 451and 452 thus becomes the semiconductor fin F11 protruding from a portionof the semiconductor substrate 410 underlying the first trenches 451 and452. The first trenches 453-457 and the semiconductor fins F12-F16 arealso formed in a similar manner. The etching of the first trenches451-457 may be performed using any of a variety of substrate etchingtechniques, such as plasma etching at a variety of pressures,temperatures, and so forth. The etching technique may also etch throughthe mask layer 430 and the pad layer 420. Since the first trenches451-457 are etched using the same etching process and at the same time,the first trenches 451-457 can have substantially the same depth, i.e.,a first depth D4.

Reference is made to FIG. 18. The mask 440 in FIG. 17 is removed, forexample, using an ashing process. Next, a mask 460 is applied to thesubstrate 410 to protect some of the first trenches 451-457 alreadycreated on the substrate 410. The mask 460 may be a photoresist. Thisphotoresist is applied to an entirety of the substrate 410 and thenpatterned so that portions of the mask 460 over parts of the substrate410 with some of first trenches 451-457 already at a desired depthremain. A pattern may be applied to the mask 460, and this pattern isfor use in deepening one or more trenches already at the first depth D4to a second depth. The pattern includes openings in the mask 460 toexpose one or more trenches to be deepened.

Reference is made to FIG. 19. Second trenches 471-476 at a second depthD5, are etched into substrate 410. The second trenches 471-476 at thesecond depth D5 may be formed by additional etching of the unmaskedtrenches at the first depth D4, wherein the first depth D4 is less thanthe second depth D5. In other words, some trenches at the first depth D4are deepened to from the second trenches 417-476 at the second depth D5.Some masked trenches are not deepened, such as the first trench 452 andportions of the trenches 451 and 453. The etching of the second trenches471-476 may be performed using any of a variety of substrate etchingtechniques, such as plasma etching at a variety of pressures,temperatures, and so forth. Since the second trenches 471-476 are etchedusing the same etching process and at the same time, the second trenches471-476 can have substantially the same depth, i.e. the second depth D5.

Reference is made to FIG. 20. The mask 460 in FIG. 19 is removed, forexample, using an ashing process. Next, a mask 480 is applied to thesubstrate 410 to protect some of the first trenches 451-453 and secondtrenches 471-476 already created on the substrate 410. The mask 480 maybe a photoresist. This photoresist is applied to an entirety of thesubstrate 410 and then patterned so that portions of the mask 480 overparts of the substrate 410 with some trenches 451-453 and 471-476already at a desired depth remain. A pattern may be applied to the mask480, and this pattern is for use in deepening one or more trenchesalready at the second depth D5 to a third depth. The pattern includesopenings in the mask 480 to expose one or more trenches to be deepened.

Reference is made to FIG. 21. Third trenches 491 and 492 at a thirddepth D6, are etched into substrate 410. The third trenches 491 and 492at the third depth D6 may be formed by additional etching of theunmasked trenches at the second depth D5, wherein the second depth D5 isless than the third depth D6. In other words, some trenches at thesecond depth D5 are deepened to from the third trenches 491 and 492 atthe third depth D6. Some masked trenches are not deepened and can bereferred to as non-deepened trenches. The etching of the third trenches491 and 492 may be performed using any of a variety of substrate etchingtechniques, such as plasma etching at a variety of pressures,temperatures, and so forth. Since the third trenches 491 and 492 areetched using the same etching process and at the same time, the thirdtrenches 491 and 492 can have substantially the same depth, i.e. thethird depth D6. In some embodiments, the non-deepened trenches 475 and476 at the second depth D5 may be adjacent to the deepened trenches 491and 492 at the third depth D6, respectively. Stated differently, thesecond trench 475 and the third trench 491 deeper than the second trench475 are communicated after the mask 480 is removed. Similarly, thesecond trench 476 and the third trench 492 deeper than the second trench476 are communicated after the mask 480 is removed as well.

Reference is made to FIG. 22. The mask 480 in FIG. 21 is removed, forexample, using an ashing process. Thereafter, a dielectric feature 500is formed on the substrate 410 to cover the semiconductor fins F11-F16and to fill the trenches 451-453, 471-476 and 491-492. The dielectricfeature 500 includes a material such as silicon oxide, silicon nitride,silicon oxynitride, low-k materials, other suitable material, or anycombinations thereof, formed using suitable techniques as discussedpreviously.

Next, as shown in FIG. 23, planarization process, such as a chemicalmechanical polishing (CMP) process is performed to remove excessdielectric feature 500 outside the trenches 451-453, 471-476 and491-492. The planarization process may also remove the pad layer 420 andthe mask layer 430 such that the semiconductor fins F11-F16 are exposed.After the planarization, portions of the dielectric feature 500 fillingthe first trenches 451-453 can be referred to first shallow trenchisolations (STIs) 501-503, portions of the dielectric feature 500filling the second trenches 471-476 can be referred to as second STIs511-516, and portions of the dielectric feature 500 filling the thirdtrenches 491 and 492 can be referred to as third STIs 521 and 522. TheseSTIs can be referred to as isolation structures in some embodiments.

The planarization process may reduce the first, second and third depthsD4, D5 and D6 to first, second and third depth D4′, D5′ and D6′,respectively. That is, after the planarization process, the firsttrenches 451-453 have reduced first depth D4′, the second trenches471-476 have reduced second depth D5′, and the third trenches 491 and492 have reduced third depth D6′. The first STIs 501-503 filling thefirst trenches 451-453 may have substantially the same thickness, whichis substantially equal to the first depth D4′. The second STIs 511-516filling the second trenches 471-476 may have substantially the samethickness, which is substantially equal to the second depth D5′. Thethird STIs 521 and 522 filling the third trenches 491 and 492 may havesubstantially the same thickness, which is substantially equal to thethird depth D6′. Since the planarization process form a substantialplanar surface for the structure shown in FIG. 23, the reduced first,second and third depth D4′, D5′ and D6′ may satisfy: D4′<D5′<D6′, whichis similar to D4<D5<D6. Therefore, thicknesses of the third STIs 521 and522 are greater than the thicknesses of the second STIs 511-516, andthicknesses of the second STIs 511-516 are greater than the thicknessesof the first STIs 501-503 at this stage. Such thickness differences maybe advantageous to provide various isolations suitable for differentdevice regions 412-418 that have different functions.

In some embodiments, the third STIs 521 and second 515 respectivelyfilling the third trench 491 and the second trench 475 that arecommunicated with each other, and hence the third STI 521 abuts thesecond STI 515, and the third STI 521 is thicker than the second STI515. Stated differently, the third STI 521 and the second STI 515 aremonolithically connected, immediately adjacent, or in contact with toeach other, and materials other than the STIs 521 and 515 are absentbetween the STIs 521 and 515. For example, a semiconductor feature, aconductive feature or a combination thereof is absent between the STIs521 and 515. For example, the third STI 521 has a sidewall S3, and thesecond STI 515 abuts an upper region of the sidewall S3. Stateddifferently, the third STI 521 has a portion protruded from a bottomsurface of the second STI 515. This arrangement may be advantageous toreduce the distance between semiconductor devices on the adjacent deviceregions 416 and 418 using different STI thicknesses. For example, thedevice regions 416 and 418 respectively have semiconductor fins F15 andF16 thereon, and the semiconductor fins F15 and F16 are adjacent, whichmeans that an additional semiconductor fin is absent between thesemiconductor fins F15 and F16. The STI isolating the adjacentsemiconductor fins F15 and F16 may include a third STI 521 abutting thesemiconductor fin F16 and a second STI 515 abutting the semiconductorfin F15, and the STIs 521 and 515 are abutted as well.

In some embodiments, the third STIs 521 and 522 have bottom surfaces ata height different from a height of the bottom surfaces of the secondSTIs 511-516, and the bottom surfaces of the second STIs 511-516 arelocated at a height different from a height of the bottom surfaces ofthe first STIs 501-503. For example, the bottom surface of the third STI521 is located at the height lower than a height of the bottom surfaceof its neighboring second STI 515. This bottom height difference may bebeneficial to make the third STI 521 thicker than the second STI 515.Stated differently, the STI isolating the adjacent semiconductor finsF16 and F15 may include an STI W3 and a dielectric protrusion P3. Thedielectric protrusion P3 protrudes from a bottom of the STI W3, and thedielectric protrusion P3 has a width less than a width of the STI W3.The dielectric protrusion P3 protrudes from the STI W3 toward thesubstrate 410. The dielectric protrusion P3 is closer to thesemiconductor fin F16 than to the semiconductor fin F15. For example,the dielectric protrusion P3 abuts the semiconductor fin F16 and isspaced apart from the semiconductor fin F15. Stated differently, theisolation structure isolating the adjacent semiconductor fins F15 andF16 includes a first portion (i.e. STI 521) and a second portion (i.e.STI 515). The first portion is closer to the semiconductor fin F16 thanthe second portion, and the first portion is thicker than the secondportion. For example, the isolation structure isolating two neighboringfins has a stepped bottom surface. Using this configuration, a portionof the STI adjacent to the semiconductor fin F16 has a thicknessdifferent from that adjacent to the semiconductor fin F15. Thisthickness difference may be advantageous to provide suitable isolationsfor the adjacent semiconductor fins F16 and F15.

Reference is made to FIG. 24. Sacrificial layer 530 is formed on atleast the semiconductor fins F11-F16. The sacrificial layer 530 may beused for implantation screening and reduction of the channeling effectduring the subsequent implantation. The sacrificial layer 530 may be anoxide layer formed, for example, using CVD or PVD. Next, an ionimplantation process is performed to impart impurities to thesemiconductor fins F11-F16 and to form wells in the semiconductorsubstrate 410. The sacrificial layer 530 is then removed, and the STIs501-503, 511-516 and 521-522 are then recessed through an etchingprocess until upper portions of the semiconductor fins F11-F16 areexposed, resulting in recessed or lowered STIs 541-543, 551-556 and561-562 on the semiconductor substrate 410, and the resulting structureis shown in FIG. 25. As illustrated, the first STIs 501-503 are recessedto form the recessed first STIs 541-543, the second STIs 511-516 arerecessed to form the recessed second STIs 551-556, and the third STIs521 and 522 are recessed to form the recessed third STIs 561 and 562. Insome embodiments, the etching process may be a wet etching process, forexample, by dipping the substrate 410 in hydrofluoric acid (HF). Inalternative embodiments, the etching process may be a dry etchingprocess, for example, the dry etching process may be performed usingCHF₃ or BF₃ as etching gases. Since the STIs 541-543, 551-556 and561-562 are recessed using the same etching process and at the sametime, they can have top surfaces at substantially the same height atthis stage. In some embodiments, a ratio of the thickness of second STI555 to the thickness of the third STI 561 abutting the second STI 555ranges from about 1/12 to about 1/24.

Reference is made to FIG. 26, mask 570 is applied to mask or cover aportion of the substrate 410, leaving another portion of the substrate410 exposed. The mask 270 may be a photoresist. This photoresist isapplied to an entirety of the substrate 410 and then patterned so thatportions of the mask 570 over parts of the substrate 410 with recessedsecond STIs 554-556 and third STIs 561-562 remain. Recessed second STIs551 and 552 and first STIs 541-543 are exposed by the mask 270. In someembodiments, a portion of the recessed second STI 553 is covered, and aportion of the recessed second STI 553 is exposed.

The exposed STIs are then recessed through an etching process, resultingin further recessed or lowered fourth STIs 581-583 and fifth STIs591-593 on the semiconductor substrate 410, and the resulting structureis shown in FIG. 27. As illustrated, the recessed first STIs 541-543 arefurther recessed to form fourth STIs 581-583, and the recessed secondSTIs 551 and 552 are further recessed to form fifth STIs 591 and 592. Anunmasked portion of the recessed second STI 553 is further recessed toform the fifth STI 593. In some embodiments, the etching process may bea wet etching process, for example, by dipping the substrate 410 inhydrofluoric acid (HF). In alternative embodiments, the etching processmay be a dry etching process, for example, the dry etching process maybe performed using CHF₃ or BF₃ as etching gases. Since the fourth andfifth STIs 581-583 and 591-593 are recessed using the same etchingprocess and at the same time, they can have top surfaces atsubstantially the same height, and the top surfaces of the fourth andfifth STIs 581-583 and 591-593 may be located at the height lower thanthe top surfaces of the second and third STIs 553-556 and 561-562 due tothe fact that the fourth and fifth STIs 581-583 and 591-593 undergoadditional etching.

In some embodiments, a portion of the STI between the adjacentsemiconductor fins F13 and F14 is recessed, and another portion of theSTI is masked. Therefore, the resulting STIs 593 and 553 between thesemiconductor fins F13 and F14 can have different thicknesses. Inparticular, the second STI 553 is thicker than the fifth STI 593. Insome embodiments, a ratio of the thickness of fifth STI 593 to thethickness of the second STI 553 ranges from about 1/1.2 to about 1/1.6.For example, the top surface of the fifth STI 593 is located at theheight lower than a height of the second STI 553. The second STI 553 andthe fifth STI 593 are abutted, monolithically connected, immediatelyadjacent, or in contact with to each other, and materials other than theSTIs 593 and 553 are absent between the STIs 593 and 553. For example, asemiconductor feature, a conductive feature or a combination thereof isabsent between the STIs 593 and 553. This arrangement may beadvantageous to reduce the distance between semiconductor devices on theadjacent device regions 414 and 416 using different STI thicknesses. Forexample, the device regions 414 and 416 respectively have semiconductorfins F13 and F14 thereon, and the semiconductor fins F13 and F14 areadjacent, which means that an additional semiconductor fin is absentbetween the semiconductor fins F13 and F14. The STI isolating thesemiconductor fins F13 and F14 may include a second STI 553 abutting thesemiconductor fin F14 and a fifth STI 593 abutting the semiconductor finF13. The second STI 553 has a sidewall S4, and the fifth STI 593 abuts alower region of the sidewall S4. Stated differently, the second STI 553has a portion protruded from a top surface of the fifth STI 593. Theabutted STIs 553 and 593 having different thicknesses may providesuitable isolations for the adjacent semiconductor fins F13 and F14,which are used to form semiconductor devices having different functions.Similarly, the fourth STI 583 and fifth STI 592 having differentthicknesses may provide suitable isolations for the adjacentsemiconductor fins F12 and F13. In some embodiments, a ratio of thethickness of fourth STI 583 to the thickness of the fifth STI 592abutting the fourth STI 583 ranges from about 0.3 to about 0.8.

In some embodiments, a portion of the semiconductor fin F13 protrudesfrom the fifth STI 593 and can be referred to as a protrusion FP3, and aportion of the semiconductor fin F14 protrudes from the second STI 553and can be referred to as a protrusion FP4. The protrusions FP3 and FP4have different thicknesses due to the thickness difference between theSTIs 593 and 553.

In some embodiments, the STI isolating the adjacent semiconductor finsF13 and F14 may include an STI W4 and a dielectric protrusion P4. Thedielectric protrusion P4 protrudes from a top of the STI W4, and thedielectric protrusion P4 has a width less than a width of the STI W4.The dielectric protrusion P4 protrudes from the STI W4 in a directionfarther away from the substrate 410. The dielectric protrusion P4 iscloser to the semiconductor fin F14 than to the semiconductor fin F13.For example, the dielectric protrusion P4 abuts the semiconductor finF14 and is spaced apart from the semiconductor fin F13. Stateddifferently, the isolation structure isolating the adjacentsemiconductor fins F13 and F14 includes a first portion (i.e. STI 553)and a second portion (i.e. STI 593). The first portion is closer to thesemiconductor fin F14 than the second portion, and the first portion isthicker than the second portion. For example, the isolation structureisolating two neighboring fins has a stepped top surface. Using thisconfiguration, a portion of the STI adjacent to the semiconductor finF14 has a thickness different from that adjacent to the semiconductorfin F13. This thickness difference may be advantageous to providesuitable isolations for the adjacent semiconductor fins F13 and F14.

In some embodiments, top surfaces of the STIs 593 and 561 are located atdifferent heights, and bottom surfaces of the STIs 593 and 561 arelocated at different heights as well. Such differences may providefurther different isolations suitable for the semiconductor fins F13 andF16. In some embodiments, a height difference between the top surfacesof the STIs 593 and 561 is different from that between the bottomsurfaces of the STIs 593 and 561. Such a difference may provide furtherdifferent isolation structures having different thicknesses suitable forthe semiconductor fins F13 and F16.

FIG. 28 illustrates formation of gate structures 610, 620, 630 and 640in device regions 412, 414, 416 and 418, respectively. The gatestructures 620 and 630 have bottoms at different heights due to that theSTIs 593 and 553 respectively underlie the gate structures 630 and 630having top surfaces at different heights. For example, the bottomsurface of the gate structure 620 is lower than the bottom surface ofthe gate structure 630. Gate dielectric 612 of the gate structure 610 isformed to wrap the semiconductor fins F11 and F12 on the logic deviceregion 412. Gate electrode 614 of the gate structure 610 is then formedon gate dielectric 612. Gate dielectric 622 of the gate structure 620 isformed to wrap the semiconductor fin F13 on the HV device region 414.Gate electrode 624 of the gate structure 620 is then formed on gatedielectric 622. Gate dielectric 632 of the gate structure 630 is formedto wrap the semiconductor fins F14 and F15 on the memory device region416. Gate electrode 634 of the gate structure 630 is then formed on gatedielectric 632. Gate dielectric 642 of the gate structure 640 is formedto wrap the semiconductor fin F16 on the CMOS image sensor region 418.Gate electrode 644 of the gate structure 640 is then formed on gatedielectric 642. Gate dielectrics and electrodes may include suitablematerials formed by suitable techniques as discussed previously.Source/drain regions for the semiconductor fins F11-F16 can be formedbefore or after the formation of the gate structures 610-640, forexample, using a suitable technique, such as implant processes orcombinations of epitaxy processes and implant processes. As a result,semiconductor devices can be formed on the device regions 412-418.

Some embodiments of the present disclosure may provide isolationstructures having different thicknesses for different semiconductordevices. Such a thickness difference may thus be advantageous to providevarious isolations suitable for different semiconductor devices. Forexample, isolation structure having suitable thickness for logic devicesmay improve their performance and reduce leakage currents, isolationstructure having suitable thickness for CMOS image sensor may reducedark currents and white pixels and may improve signal to noise ratio(SNR), and isolation structure having suitable thickness for memorydevices may improve their data retention. Moreover, the isolationstructures having different thicknesses are abutted and betweensemiconductor devices. This arrangement may be advantageous to reducethe distance between semiconductor devices using isolation structureswith different thicknesses.

According to some embodiments, a structure includes a semiconductorsubstrate, a first fin, a second fin, a first isolation structure, and asecond isolation structure. The semiconductor substrate has a memorydevice region and a logic core region. The first fin is in the memorydevice region of the semiconductor substrate. The second fin is in thelogic core region of the semiconductor substrate. The first isolationstructure is around the first fin. The second isolation structure isaround the second fin, and a thickness of the first isolation structureis different from a thickness of the second isolation structure.

According to some embodiments, a structure includes a semiconductorsubstrate, a first fin, a second fin, a first isolation structure, and asecond isolation structure. The semiconductor substrate has a logic coreregion and a complementary metal-oxide-semiconductor (CMOS) image sensorregion. The first fin is in the logic core region of the semiconductorsubstrate. The second fin is in the CMOS image sensor region of thesemiconductor substrate. The first isolation structure is around thefirst fin. The second isolation structure is around the second fin, anda thickness of the first isolation structure is different from athickness of the second isolation structure.

According to some embodiments, a structure includes a semiconductorsubstrate, a first fin, a second fin, a first isolation structure, and asecond isolation structure. The semiconductor substrate has a memorydevice region and a high voltage (HV) device region. The first fin is inthe memory device region of the semiconductor substrate. The second finis in the HV device region of the semiconductor substrate. The firstisolation structure is around the first fin. The second isolationstructure is around the second fin, and a thickness of the firstisolation structure is different from a thickness of the secondisolation structure.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A structure, comprising: a semiconductorsubstrate having a first region and a second region; a base portionextending from the second region of the semiconductor substrate with afirst height, wherein the base portion has opposite sidewalls; a firstfin extending from the first region of the semiconductor substrate witha second height; a second fin extending from the base portion with athird height, wherein a bottom of a sidewall of the first fin and abottom of a sidewall of the second fin are at different heights, thebase portion is wider than the second fin; a first isolation structurearound the first fin; and a second isolation structure around the secondfin and in contact with the sidewalls of the base portion, wherein athickness of the first isolation structure is different from a thicknessof the second isolation structure, and a top of the first isolationstructure is higher than a top of the second isolation structure.
 2. Thestructure of claim 1, wherein the thickness of the first isolationstructure is greater than the thickness of the second isolationstructure.
 3. The structure of claim 1, wherein a bottom of the firstisolation structure is lower than a bottom of the second isolationstructure.
 4. The structure of claim 1, wherein the first isolationstructure is spaced apart from the second isolation structure.
 5. Thestructure of claim 1, wherein the second isolation structure has astepped bottom surface.
 6. The structure of claim 1, further comprising:a third isolation structure abutting the second isolation structure,wherein a thickness of the third isolation structure is different fromthe thickness of the second isolation structure.
 7. The structure ofclaim 6, wherein the semiconductor substrate further has a high voltage(HV) device region and the structure further comprises: a third fin inthe HV device region of the semiconductor substrate, wherein the thirdisolation structure is around the third fin.
 8. The structure of claim6, wherein the thickness of the third isolation structure is greaterthan the thickness of the second isolation structure.
 9. The structureof claim 1, wherein the bottom of the sidewall of the first fin is lowerthan the bottom of the sidewall of the second fin.
 10. A structure,comprising: a semiconductor substrate having a first region and a secondregion; a first fin extending from the first region of the semiconductorsubstrate with a first height; a second fin extending from the secondregion of the semiconductor substrate with a second height; a third finadjacent the second fin and extending from the semiconductor substratewith a third height, wherein the third height of the third fin isgreater than the first height of the first fin and less than the secondheight of the second fin; a first isolation structure around the firstfin; and a second isolation structure around the second fin, wherein athickness of the first isolation structure is different from a thicknessof the second isolation structure, wherein a bottom surface of thesecond isolation structure has a first portion extending laterally froma bottom of a sidewall of the second fin and a second portion extendinglaterally from a bottom of a sidewall of the third fin, the first andsecond portions of the bottom surface of the second isolation structureare at different heights, and a top of the second isolation structure ishigher than a top of the first isolation structure.
 11. The structure ofclaim 10, wherein the thickness of the second isolation structure isgreater than the thickness of the first isolation structure.
 12. Thestructure of claim 10, wherein a third isolation structure abutting thefirst isolation structure, and a bottom of the third isolation structureis lower than a bottom of the first isolation structure.
 13. Thestructure of claim 10, wherein the first portion of the bottom surfaceof the second isolation structure is lower than the second portion ofthe bottom surface of the second isolation structure.
 14. The structureof claim 10, wherein a bottom of a sidewall of the first fin is higherthan the bottom of the sidewall of the second fin.
 15. The structure ofclaim 10, wherein the first portion of the bottom surface of the secondisolation structure and the second portion of the bottom surface of thesecond isolation structure have different widths.
 16. A structure,comprising: a semiconductor substrate; a first fin over thesemiconductor substrate; a second fin over the semiconductor substrate,wherein a width of the first fin is different from a width of the secondfin; a third fin over the semiconductor substrate, wherein a bottom ofthe first fin, a bottom of the second fin, and a bottom of the third finare at different heights; a first isolation structure around the firstfin; and a second isolation structure around the second fin, wherein athickness of the first isolation structure is different from a thicknessof the second isolation structure, and a top of the first isolationstructure is higher than a top of the second isolation structure. 17.The structure of claim 16, further comprising: a third isolationstructure around the third fin, wherein a bottom of the third isolationstructure is higher than a bottom of the first isolation structure. 18.The structure of claim 16, wherein a top of the first fin and a top ofthe second fin are at substantially the same height.
 19. The structureof claim 17, wherein the top of the second isolation structure is at aheight different from that of a top of the third isolation structure.20. The structure of claim 17, wherein a top of the third isolationstructure has a step profile.